ipmacsec: fix warning on 32bit platform

ipmacsec: fix warning on 32bit platform

On some 32 bit platforms, the printf was causing warning:
ipmacsec.c: In function ‘getattr_u64’:
ipmacsec.c:655:47: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘unsigned int’ [-Wformat=]
   fprintf(stderr, "invalid attribute length %lu\n",

Resolve by computing length as size_t first.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>